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Romain
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Table salt heating bath?
Hi,
I want a way to heat a medium sized round bottom flask (100-500 ml) for a future dry distillation of acetic acid from a mix of NaHSO4 and
CH3COONa. This reactions involves rather high temperatures (I don't know precisely how high, maybe around 200 or 300°C) I already own a
hotplate stirrer and I don't want to spend money on a heating mantle so I considered using heating baths.
Apparently people have used sand, oils (cooking, silicone, mineral), paraffin wax, steel or copper shot, molten metals, molten salts, ... But I never
heard of a table salt bath (in powder form, not a solution).
Table salt looks to me like a good substitute for a sand bath: It's cheap, doesn't fume like oil does and you don't need to buy a 50 kg bag of it...
And as a bonus, the Mohs hardness of sodium chloride is around 2 or 3, that of sand is around 6 or 7 and that of borosilicate glass is also around 6
or 7, which means glass won't get scratched by the salt bath whereas it might get scratched by a sand bath (I haven't tested).
So has anyone any experience with table salt baths? Are they ever used?
P.S. I did run a search before posting but didn't find anything relating to table salt baths.
[Edited on 29-12-2015 by Romain]
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Heavy Walter
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Hi
My limited experience with salt is that it tends to solidify, making difficult to remove a flask.
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Romain
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Thanks for your input.
Did you heat enough to melt the sodium chloride (ca. 800°C IIRC)? Why do you suspect it solidified? Was it because of impurities like anti-caking
agents, etc.? Was the salt slightly humid, causing it to dissolve in that small amount of water and crystallize around the flask?
I can't test for myself now because I don't have access to my lab unfortunately.
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Heavy Walter
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Hi
Just heating at ca. 400 °C.
And in very humid ambient. I used coarse salt and crystals tended to fuse between themselves.
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careysub
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How about calcium carbonate sand? This is widely available, many "play sands" are not silica sand. Much softer than silica.
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ariep641514
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ordinary salt of chloride seems to be reactive at high temp. it promote rust, if you not clean after use, residue remain in your tools will absorb
water , and corode your heating element, or ferous part of you equipment.
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Pyro
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I have used table salt baths often and found them to be very practical
all above information is intellectual property of Pyro.  |
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Romain
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Ok thanks for your replies! I'll be sure to test it when I can.
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subsecret
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But the salt can be easily washed off with water if it sticks. For a really good bath, I'd recommend copper powder, though I've never tried it myself.
It'd be like a moldable heating block.
Fear is what you get when caution wasn't enough.
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Romain
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I've thought about copper powder bath too but I don't have any copper powder. I do have about 1.5kg worth of 400 mm^2 stranded copper cable though, so
if I manage to cut it in small pieces, it could work for a heating bath.
I just ran a quick calculation: 1 kg of copper powder in the form of small spheres, when packed, takes up about 170 cm^3. (Density of copper: 9 g/cm^3; when packed (bulk density): 64%*9 = 5.75 g/cm^3). This means it's going to be quite
costly to have a big enough copper powder bath. But I'll give it a try if I ever get around to chipping that copper cable.
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careysub
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Quote: Originally posted by Romain  | I've thought about copper powder bath too but I don't have any copper powder. I do have about 1.5kg worth of 400 mm^2 stranded copper cable though, so
if I manage to cut it in small pieces, it could work for a heating bath.
I just ran a quick calculation: 1 kg of copper powder in the form of small spheres, when packed, takes up about 170 cm^3. (Density of copper: 9 g/cm^3; when packed (bulk density): 64%*9 = 5.75 g/cm^3). This means it's going to be quite
costly to have a big enough copper powder bath. But I'll give it a try if I ever get around to chipping that copper cable. |
Try aluminum instead. Much lighter (1/3 the density) and much cheaper. It has half the thermal conductivity of copper, but that is still very high -
the thermal conductivity of aluminum is 70 times higher than quartz.
Cut aluminum wire pieces are available for cheap on eBay.
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Romain
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Aluminium is a good idea! I couldn't find the cut wire pieces you were talking about on eBay, do you have any specific keywords for a search?
I could also just melt some scrap aluminium and make shot by pouring the molten metal in a bucket of water, but its more troublesome than buying it,
considering I don't have a furnace.
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DraconicAcid
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| Quote: Originally posted by Romain | Aluminium is a good idea! I couldn't find the cut wire pieces you were talking about on eBay, do you have any specific keywords for a search?
I could also just melt some scrap aluminium and make shot by pouring the molten metal in a bucket of water, but its more troublesome than buying it,
considering I don't have a furnace. |
Molten aluminum in water??? Sounds like a very bad idea, at least based on the warnings I got about using water on an aluminum fire at the Norsk
Hydro plant.
ETA: https://www.youtube.com/watch?v=29O-8LckmiU
Apparently not so much.
[Edited on 2-1-2016 by DraconicAcid]
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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blogfast25
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Nah. Not a problem at all. Been there, done that.
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BromicAcid
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Thermal conductivity of sodium chloride at 289K is 6.5 W/(mK). Silicon dioxide is 1.3-1.5 W/(mK). For comparison, copper is 385.0. But in each of
these cases (table salt, sand, copper shot) you also have void space between the particles, this reduces the thermal conductivity even further. What
I am getting at here is that even through you might see sand in a literature reference, a lot of hot plates that still have their safety information
recommend against it because it is such a good insulator that you can overheat your coil. That being said, at least sodium chloride is a bit more
conductive than sand but compared to copper the difference is negligible. Simply said, the same advice on a sand bath (be careful of using too high
of a setting or heating too quickly) will apply to your salt 'bath'.
You could also compare specific heat to give yourself a better idea of possible performance differences.
Interestingly enough, all three of these are better strictly speaking than a mineral oil bath which only has a thermal conductivity of 0.162 W/(mK).
Off the top of my head I would say however that because liquids can undergo convection movement and can stir you will end up with much better heat
transfer than relying on thermal conductivity alone.
I didn't see it mentioned above so I thought I would mention it here.
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JJay
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I've used sand baths on a hot plate before, and almost every time I've tried it, the glass has broken due to thermal stress. I definitely don't
recommend it.
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careysub
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| Quote: Originally posted by Romain | Aluminium is a good idea! I couldn't find the cut wire pieces you were talking about on eBay, do you have any specific keywords for a search?
I could also just melt some scrap aluminium and make shot by pouring the molten metal in a bucket of water, but its more troublesome than buying it,
considering I don't have a furnace. |
Yes, the are calling them "aluminum pellets" and the vendor is "handi-ramp510".
They sell them with two different price+shipping options, a 10 lb box with free shipping for $100 (thus $10/lb); OR you can use a different listing
they have that sells them for $5.00 lb, after $12.50 for the first lb. This second one may not come up on a search because this listing does not have
the word "aluminum" in the title.
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careysub
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| Quote: Originally posted by BromicAcid |
Interestingly enough, all three of these are better strictly speaking than a mineral oil bath which only has a thermal conductivity of 0.162 W/(mK).
Off the top of my head I would say however that because liquids can undergo convection movement and can stir you will end up with much better heat
transfer than relying on thermal conductivity alone. |
Convection will transfer heat orders of magnitude faster than conduction.
The thermal conductivity of the fluid is uninformative, the efficiency of convection depends on the "heat transfer coefficient" which is actually a
combination of conductivity, viscosity, heat capacity, and the geometry of the situation.
A fundamental problem with any sort of powder bath is that conduction slows down dramatically with increasing thickness of the insulator (lets face
it, it is better description than "conductor" for sand), and necessarily creates a large temperature gradient (did I hear your flask breaking?). I
think sand baths are likely only really good for microscale work (thinner layers are better).
Convective baths always keep the bath temperature nearly uniform (which is why Thiele tubes work).
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Romain
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Ok, I tried the salt bath and it doesn't work very well.
I tried distilling water from a 250 ml RBF and the hotplate temperature was set to 200°C. The bath consisted of a 115 mm glass crystallizing dish
full of fine salt. The bottom of the flask was about 10 mm above the bottom of the dish and was surrounded with salt. After 1 hour, the bath hardly
reached 100°C at the surface and was at more than 300°C at the bottom!
This means that the hotplate can't regulate it's temperature properly with a salt bath on top of it. It can get much hotter that it is supposed to and
this could destroy the heating element.
The heating is also quite inefficient: it took around 1 hour of heating to start boiling 150 ml of water.
Basically, the salt bath is a bad idea: it is slow and could destroy your equipment.
However, I discovered that salt is pretty useful to clean flasks with gunk in them. I tried distilling nitrocellulose lacquer because it contained
toluene after the distillation was complete, the nitrocellulose started precipitating inside the flask. after some manual scrubbing I was left with
some nitrocellulose residue inside the 500 ml Erlenmeyer flask I used and could not reach inside with a brush. So I poured in around 100 ml of ethanol
and some coarse salt, stoppered the flask and shook for a while. The salt scrubbed the residue and my flask was perfectly clean after that!
I hope these results will be useful to someone.
[Edited on 15-1-2016 by Romain]
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stoichiometric_steve
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That's actually quite a nice idea, thank you!
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XeonTheMGPony
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http://www.ebay.com/itm/10-LBS-COPPER-FINE-POWDER-99-purity-...
Was looking at copper powder as an idea and found it is pretty cheap.
Oil baths are messy but keep very uniform heat, especially if stirred, Other ones I have used is brine baths (Water super saturated with salt).
I have made super cheap mantles by molding plaster of parris with rope wrapped around the flask, then modified broken hair driers for the NiChrom
coils with good success.
To expand the coil to meet the voltages you'll be using heat with a torch till red then stretch it out. I used mostly 12V as the heating current and a
salt water rheostat.
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careysub
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Thanks, that is pretty informative. Good ideas.
In evaluating a copper dust bath (325 mesh is pretty fine) vs an aluminum shot (or cut wire) bath consider that the cheap copper dust here is $12 a
pound with a density of around 6 (using the fact that fine sand is about 75% the density of quartz). This is $158/liter.
Aluminum shot can be had on eBay for ~$6/lb, and has a density of around 1.73 (2.71 * 0.64 sphere random packing efficiency). This translates to
$23/liter. The packing efficiency for the cut wire pieces is probably a bit lower than spheres, so this may be a trifle high.
And the copper bath will be very heavy.
Can you post a picture of one of your plaster mantles?
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Sulaiman
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I had considered using plaster of paris to make a heating mantle (using nichrome wire)
but when I read the Wikipedia article https://en.wikipedia.org/wiki/Plaster
I thought that it would crumble when the anhydrite is formed above 130C
What maximum temperature have you run plaster of paris at ?
P.S. I happen to have copper powder from galvanic cell experiments and I have been keeping it in an airtight bottle
... would it not corrode in air, especially when heated, forming thermally insulating surfaces ?
... being fairly dense, I imagine VERY buoyant flasks
[Edited on 20-1-2016 by Sulaiman]
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careysub
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Plaster of paris is commonly used in metal casting molds, and moderate temperature furnaces - but you don't use pure plaster.
Typically 50% fine sand is added, you can also use perlite in whole or part for a lighter, more insulating product. Incorporating glass fiber (as
loose wool, chopped fiber, or cloth) can make it even more durable. And the plaster is usually baked in an oven (450-500 F) for a few hours to dry
after it is fully cured (3 days).
Perlite and furnace cement can be used for something that is really high temperature (far more than is needed in this application).
And I imagine getting a cheap metal bowl and casting the plaster or cement mix into it so that the bowl is a permanent part of heating mantle will
ensure no crumbling or break-up problems ever occur.
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XeonTheMGPony
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| Quote: Originally posted by Sulaiman | I had considered using plaster of paris to make a heating mantle (using nichrome wire)
but when I read the Wikipedia article https://en.wikipedia.org/wiki/Plaster
I thought that it would crumble when the anhydrite is formed above 130C
What maximum temperature have you run plaster of paris at ?
P.S. I happen to have copper powder from galvanic cell experiments and I have been keeping it in an airtight bottle
... would it not corrode in air, especially when heated, forming thermally insulating surfaces ?
... being fairly dense, I imagine VERY buoyant flasks
[Edited on 20-1-2016 by Sulaiman] |
Yes they disintegrate, basically only used for the very rare occasion I need high temps, the NiChrom is stored in hard card board tubes. just a very
fast cheap methode when you don't need it enough to pay high dollar. they do hold up for more then one flask though.
Andabove post states how one could make it more perminant
[Edited on 28-1-2016 by XeonTheMGPony]
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